Optical fiber cables have been widely deployed mainly as trunk lines and rings around cities. The design of such cables was based on long haul or urban loop requirements and optical fiber counts were high, i.e. 24 to 864 and higher fiber counts. Because long distance applications were most common, emphasis was based upon minimizing signal loss. Other factors considered in the designs were the ease of management of large numbers of fibers during access, splicing and storage. Generally, access to a single fiber was not involved because usually splices were made at the terminations of the cable or the splicing involved the branching off of several optical fibers.
One common optical fiber cable design is known as stranded loose tube cable. See, for example, U.S. Pat. No. 5,390,273. In such cables, there is a central strength member around which a plurality of plastic tubes, each loosely receiving a plurality of optical fibers, are wound either helically or in S-Z fashion. The tubes and strength member are encircled by a jacket or sheath which can comprise one or more layers of plastic or of plastic and metal. Such cable provides good protection for the fibers, and when the fiber count is high, organization of the fibers is facilitated since the fibers are distributed among several tubes, e.g. six tubes. However, when the fiber count is low, e.g. two to twelve fibers, multiple tubes are not desirable, not only for cost reasons, but also because for a given cable size, the space within the cable is not efficiently used.
Another common optical fiber cable design is known as a central loose tube cable. See, for example, U.S. Pat. No. 5,509,097. In such cables, a centrally disposed tube, e.g. made of plastic, loosely receives a plurality of optical fibers, and to provide resistance to tensile and contraction forces, strength members are disposed radially outwardly of the tube and usually within a jacket or sheath. The strength members can be yarns, which are not effective for resisting contraction forces, or relatively rigid rods which resist both tensile and contraction forces.
The central loose tube design provides less protection for the optical fibers than the stranded loose tube design, but can be smaller in size and the fibers are easier to access. However, substantial handling of the cable is required to make the fibers available for splicing or connection to other devices, all of the fibers are exposed when the tube is opened which is not desirable if a splice connection to only a single optical fiber is desired, identification of groups is more difficult since the fibers are in one tube and the cable can be less flexible due to the location of the strength members.
Another cable design which is less common in the United States is known as a slotted core cable. See, for example, U.S. Pat. No. 5,193,134. In such cables, there is a core comprising a central strength member encircled by a layer of plastic having a radial thickness sufficient to permit the formation of longitudinal slots or grooves of a size which will loosely receive a plurality of optical fibers in each slot or groove. The slots open outwardly of the core and are closed in the finished cable by a jacket or sheath to form ducts for the optical fibers. Such cables have characteristics similar to the central loose tube cables, but they are difficult to manufacture due to the fact that the optical fibers are fed by fiber pay-offs into the preformed slots. The pay-offs must follow accurately the slots in the core as it is advanced during the feeding of the fibers into the slots while maintaining precise tension control on the fibers. Such conditions can be especially difficult if the slots are S-Z slots, or alternating direction slots which, as is known in the art, are desirable for mid-span access to the fibers.
One of the problems with the slotted core cable when it is to be used for a cable with only a few optical fibers is that only a small reduction in the diameter of the core can be made. Thus, the layer of plastic around the strength member must have a thickness which will not only provide slots of a radial depth and circumferential width sufficient to loosely receive the fibers, but also provide a circumferentially continuous plastic portion inwardly of the slots to prevent separation of the sidewalls of the slots and to maintain the spacing of such sidewalls. For the latter reason, the plastic layer has a minimum radial thickness which is greater than the radial thickness required to provide slots for the fibers.
Because of the tubes, known as buffer tubes, or the slots used, in the designs described hereinbefore, to protect the optical fibers, such designs do not use “tight buffered” fibers. As used herein, “tight buffered” means an optical fiber having, in addition to the layers commonly applied during the manufacture of the fiber, a contacting layer or layers of protective material, such as a polymeric material. Such protective layer or layers increase the outer diameter of the fiber from a typical value of 0.25 mm to from about 0.6 mm to about 0.9 mm. Tight buffered fibers are well known in the art, and because of the tightly encircling layer or layers, the fibers can be handled without further protection, such as buffer tubes or a protective sleeve on an exposed length of fiber.
In all of the cable designs described hereinbefore, the cable usually includes a plastic outer jacket. In order to provide the necessary physical characteristics for the jacket, normally, it must have a radial thickness which is greater than the diameter of an optical fiber.
To provide high speed access of a user's equipment, e.g. at the user's home or business, to the optical fiber cables already installed as trunk or feeder cables, there is a demand for a simple, relatively inexpensive and easy to manufacture cable which can be used to connect the user to the large capacity, high speed cables described hereinbefore. Such a cable is often called a fiber to the home (FTTH) cable. An FTTH cable usually does not need to have more than 12 optical fibers and can have only one optical fiber and typically, is relatively short so that signal loss is not a significant factor. However, an FTTH cable should have the other characteristics of the larger, long distance cables such as ease of handling, ease of access to the fibers, ease of connection of the fibers to other devices, adequate protection of the fibers, flexibility and be adapted to withstand the temperatures of the outdoors. Also, access to a single optical fiber should be convenient without impairing the safety of other optical fibers in the cable. It is known from U.K. Patent Application GB 2,114,771A to provide longitudinally extending compartments in a jacket for loosely receiving an optical fiber, or a bundle of optical fibers, in each compartment. However, the cable disclosed therein is not satisfactory as an FTTH cable not only because the jacket is bonded to the strength member or is integral with a layer of rubber or plastics encircling the strength member so that the jacket cannot be easily separated manually from the strength member, but also because the location of the fiber receiving compartments cannot be readily determined from externally of the cable.
It is apparent from the foregoing description of prior art cables that the foregoing long distance cables cannot provide the characteristics desired for an FTTH cable. The use of a central strength member can provide the strength and flexibility requirements, but if provided with the plastic layer of a size needed for a stranded loose tube or slotted core cable, the diameter of the cable is unnecessarily large and excess plastic material is required. It should be borne in mind that the element used as the core for the strength member need be only about 3–5 mm in diameter and any plastic layer, or “upjacketing”, around the element can be omitted if it does not contact the optical fibers, if it does not provide slots for receiving the optical fibers or if it does not provide a form for receiving buffer tubes. If the fibers are to contact the strength member (core element plus plastic layer), the plastic layer can be relatively thin as compared to the thickness of a plastic layer for a loose tube or slotted core cable.